In process automation technology, sensors are used in a multitude of applications for registering various measured variables. The signals of the sensors are fed to measurement transmitters, where they are suitably conditioned and displayed or forwarded via a fieldbus to a superordinated unit. Examples which can be named include pressure and temperature transmitters, pH/redox-potential measurement transmitters, conductivity transmitters, etc., for the corresponding measured variables, pressure, temperature, pH-value and conductivity value, respectively.
The task of a sensor is to convert the physical/chemical, measured variable into an electrical signal. In the measurement transmitter, a first conditioning of the analog, electrical signal occurs. Then, such is converted into a digital signal, which is fed to a microcontroller, in which the actual signal processing takes place.
The circuit needed for the analog signal conditioning comprises, as a rule, a circuit board with a plurality of analog components. In the manufacture of such a signal conditioning circuit, populating errors are not out of the question. Here, one distinguishes, in principle, two cases. First, the pertinent component is wrong. I.e., the wrong component was used in the populating. Second, the pertinent component is missing. I.e., the component was forgotten in the populating.
Both defect possibilities have, as a rule, a significant effect on quality of the signal processing and on measurement result.
An opportunity for preventing such populating errors is to perform a test, e.g. an ICT (in-circuit test), following the populating of the circuit board. For this, appropriate test points must be provided on the circuit board.
By applying defined alternating voltage signals and tapping resulting signals at individual test points, an analog signature analysis (ASA) can be performed. In the case of known impedance of the components, or component groups, defective components can be detected from the current-voltage characteristic, which is referred to as the “impedance signature”.
Due to ever-increasing component density on circuit boards, suitable test points are frequently no longer available. I.e., such circuit boards are not suited for ICT-tests.
Most often, for ICT-tests, complicated test instruments with installed signal converters and complicated signal processing are required for registering and calculating the impedances. The calculated actual impedance is compared in a test program with what the impedance should be, and a deviation calculated. If the deviation is too large, the pertinent component is reported as defective.